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Fair Value Measurements - Fair Value Hierarchy for Financial Assets (Details) - USD ($)
$ in Millions
Sep. 30, 2024
Dec. 31, 2023
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Cash equivalents $ 315.4 $ 315.9
Debt securities, available-for-sale 1,871.1 2,157.8
Other assets 21.2 15.2
Total assets measured at fair value on a recurring basis 2,207.7 2,488.9
U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 1,038.1 1,612.5
Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 391.1 184.7
Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 441.9 360.6
Level 1    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Cash equivalents 315.4 315.9
Debt securities, available-for-sale 0.0 0.0
Other assets 21.2 15.2
Total assets measured at fair value on a recurring basis 336.6 331.1
Level 1 | U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 0.0 0.0
Level 1 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 0.0 0.0
Level 1 | Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 0.0 0.0
Level 2    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Cash equivalents 0.0 0.0
Debt securities, available-for-sale 1,871.1 2,157.8
Other assets 0.0 0.0
Total assets measured at fair value on a recurring basis 1,871.1 2,157.8
Level 2 | U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 1,038.1 1,612.5
Level 2 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 391.1 184.7
Level 2 | Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 441.9 360.6
Level 3    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Cash equivalents 0.0 0.0
Debt securities, available-for-sale 0.0 0.0
Other assets 0.0 0.0
Total assets measured at fair value on a recurring basis 0.0 0.0
Level 3 | U.S. government agencies    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 0.0 0.0
Level 3 | Commercial paper    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale 0.0 0.0
Level 3 | Corporate debt    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]    
Debt securities, available-for-sale $ 0.0 $ 0.0